Microsoft released Azure Active Directory Connect version 1.1.553.0 on June 26, 2017. More importantly, they published an important security advisory one day later.
Microsoft Security Advisory 4033453 - Vulnerability in Azure AD Connect Could Allow Elevation of Privilege explains,
The [ADD Connect version 1.1.553.0] update addresses a vulnerability that could allow elevation of privilege if Azure AD Connect Password writeback is misconfigured during enablement. An attacker who successfully exploited this vulnerability could reset passwords and gain unauthorized access to arbitrary on-premises AD privileged user accounts. The issue is addressed in the latest version (1.1.553.0) of Azure AD Connect by not allowing arbitrary password reset to on-premises AD privileged user accounts.Microsoft highly recommends all customers update to version 1.1.553.0 or later to mitigate this vulnerability, even if you don't use the optional password writeback feature. If you are unable to update immediately, the article above describes mitigation steps you can consider.
- If the AD DS account is a member of one or more on-premises AD privileged groups, consider removing the AD DS account from the groups.
- If an on-premises AD administrator has previously created Control Access Rights on the adminSDHolder object for the AD DS account which permits Reset Password operation, consider removing it.
- It may not always be possible to remove existing permissions granted to the AD DS account (for example, the AD DS account relies on the group membership for permissions required for other features such as Password synchronization or Exchange hybrid writeback). Consider creating a DENY ACE on the adminSDHolder object which disallows the AD DS account with Reset Password permission using Windows DSACLS tool.
Besides this important security update, AAD Connect 1.1.553.0 includes several fixes, new features, and improvements both in AAD Connect and AD FS management. Read the Azure AD Connect: Version release history for a complete list.
With Azure AD Connect being such an important part of your cloud connectivity and authentication solution, it's super important to stay on top of any updates.